The activated sludge process is a biological process for the treatment of wastewater. Advanced processes comprise a sequence of oxic, anoxic, and anaerobic process phases in mixed liquor suspended solids that correspond to metabolic processes of living organisms. The process phases can be controlled by controlling the oxygen available in the wastewater.
Aeration is provided to the wastewater during the oxic phase, in which dissolved oxygen is present in the wastewater. Carbonaceous compounds are oxidized. Ammonium is oxidized to nitrates and nitrides. The organisms take up phosphates from the wastewater for storage in an energy rich form as polyphosphates.
The anoxic phase is characterized by the absence of dissolved oxygen. Aeration is turned off. The organisms use chemically bound oxygen in the form of nitrates and nitrides for respiration. Nitrates and nitrides are converted to nitrogen and carbonaceous compounds are oxidized.
In the anaerobic phase, aeration remains off and no dissolved oxygen is present. The organisms have used the available chemically bound oxygen in the anoxic phase. Accordingly, in the anaerobic phase, the organisms metabolize the previously stored energy rich form of phosphate and release phosphate back into the wastewater as orthophosphate.
U.S. Pat. Nos. 4,333,838; 4,859,341; 5,076,928; and 5,242,592 describe various apparatus and processes for treating wastewater with activated sludge. U.S. Pat. No. 5,076,928 describes a process for biological wastewater treatment in which the oxic, anoxic, and anaerobic reactions occur sequentially over the entire volume of a single reactor. The turbidity of the wastewater is monitored to provide a virtually instantaneous indication of the progress of the reaction through the separate phases and for control of aeration. Blowers are activated at maximum turbidity to initiate the oxic phase and are deactivated at minimum turbidity to initiate the anoxic and subsequent anaerobic phase. Up to 75% of phosphates can be removed by the process described in U.S. Pat. No. 5,076,928. Greater phosphate removal requires the addition of chemical flocculents either to the effluent or directly to the activated sludge tank. For smaller plants, a constant quantity of coagulant is maintained in the wastewater of the plant for elimination of the calculated, globally remaining phosphate content. For larger plants, a controlled coagulant dosage is preferred.
U.S. Pat. No. 5,242,592 describes another improvement in the activated sludge process in which the oxic phase is initiated independent of turbidity if the phosphate concentration exceeds a maximum set point in the anaerobic phase before the turbidity has reached a maximum. Acetic acid is added to increase phosphate elimination at the end of the anaerobic phase, prior to switching on the supply of oxygen.
The efficiency with which nitrogen is eliminated in the activated sludge process is dependent upon the ratio between nitrogen and the organic load, or biological oxygen demand, that is present in the wastewater. Biological oxygen demand, or BOD, can be determined after a five day period, and is designated "BOD5." The parameter BOD5 corresponds to the carbonaceous organic load in the wastewater that is fed to the activated sludge plant. The ratio of total incoming nitrogen to BOD5 should be less than or equal to 0.18 for efficient denitrification of wastewater. The ratio is determined based on the grams of incoming nitrogen and BOD per capita day. For example, a ratio of 11 g N/cap.d to 60 g BOD/cap.d=0.18 is commonly encountered.
However, values greater than 0.18 result in reduced denitrification efficiency because of the lack of organic load in the influent to the plant. Values greater than 0.25 result in instability in the wastewater treatment plant. If it is not possible to normalize the nitrogen to BOD5 ratio in the influent, then a BOD substitute is required.
Controlling the nitrate nitrogen to BOD5 ratio has become of increasing importance because activated sludge plants have been used to expand the treatment of trickling filter plants. Wastewater effluent from the trickling filter plant is presented to the activated sludge plant with a deficit in biological oxygen demand that is due to the reduced organic load of the trickling filter plant effluent. Trickling filter plants are capable of virtually complete nitrification of organic compounds. However, denitrification typically does not occur because of the usually low retention time of the wastewater in the trickling filter plant. Trickling filter plants typically have to be expanded today because of the ever increasingly stringent effluent requirements for denitrification of wastewater.
Expensive nitrate monitoring systems that require extensive and expert maintenance have been used to monitor the incoming nitrate load to activated sludge plants. When an excess of nitrate is measured, then a carbonaceous organic material or its salt is added to the influent to the activated sludge plant to make up for the deficit of organic material. The carbonaceous organic material is added to obtain the ratio of nitrate nitrogen to BOD5 of 0.18.
Commonly used carbonaceous organic materials include easily biodegradable substrates such as methanol, acetic acid, acetyls, or similar carbon carriers. The carbonaceous organic material typically is added directly to the activated sludge tank during the anoxic phase, whether the process is conducted in a single tank or in multiple tanks. The addition can take place either uncontrolled with empirically determined quantities or in a controlled manner by monitoring the nitrate levels using nitrate monitoring systems.
However, nitrate monitoring systems are expensive and require extensive and expert maintenance. It would be desirable to operate activated sludge plants for efficient denitrification of influents with a deficit of organic load without the use of such expensive and troublesome apparatus. Accordingly, it is an object of this invention to provide an activated sludge process that can efficiently denitrify wastewater influents that have a deficit of organic load without the use of expensive nitrate monitoring apparatus.